Atomic resonance gas cell having an evacuated double end wall structure



May 5, 1970 G. R. HUGGETT 3,510,758

ATOMIC RESONANCE GAS CELL HAVING AN EVACUATED D END WALL STRUCTURE Filed Nov. 14, 1967 OUBLE Z 25 j 0 O O O O O O O O O O 3 '3' s I9 38 :l: 85:1: A PHASE DETECTOR OJ-'- Z I A (56 Z Z Z Z Z Z Z Z Z |07Hz- MODULATOR FREQUENCY 5MH MULT'PL'ER MULT'PL'ER SYNTHESIZER OSCILLIZHOR I 5MHZ DIVIDER MHZ MENTOR GEORGE R. HUGGETT I BY 4 DIVIDER 1|00KHZ v ATTORNEY United States Patent 01 3,510,758 ATOMIC RESONANCE GAS CELL HAVING AN EVACUATED DOUBLE END WALL STRUCTURE George R. Huggett, Sunnyvale, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Nov. 14, 1967, Ser. No. 682,904 Int. Cl. H01s 3/05 U.S. Cl. 324-.5 5 Claims ABSTRACT OF THE DISCLOSURE The gas cell includes a cylindrical side wall portion and a pair of end wall structures. The end wall structures include an inner and an outer partition with the inner partition facing the gaseous ensemble and the outer partition facing the outside atmosphere. The region between the inner and outer partitions is evacuated such that atmospheric pressure is not transmitted to the atomic vapor through the end wall structures, whereby the atomic resonance frequency of the alkali metal vapor is not substantially influenced by pressure fluctuations in the ambient atmosphere. The inner and outer wall partitions are supported essentially only at their marginal edges from the ends of the cylindrical side wall portion of the gas cell. In a preferred embodiment, the inner end wall partition is made substantially thinner than the outer partition such that if the region between the inner and outer partitions leaks up to atmospheric pressure, the innermost partition will fracture causing the gas cell to become inoperative. In this manner, a fail safe feature is provided, inasmuch as erroneous pressure shifted output frequencies are not obtainable. An illustrative atomic resonance frequency standard incorporating the double walled cell is included.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85568 (72 Stat. 435; 42 U.S.C. 2457).

DESCRIPTION OF THE PRIOR ART Heretofore, attempts have been made to reduce the pressure produced changes in the operating frequency of the frequency standard by causing the end walls of the gas cell to be outwardly domed as opposed to flat end walls, whereby the gas cell is rendered less sensitive to pressure fluctuations in the outside atmosphere. While such curved end Wall structures have reduced the pressure sensitivity of the frequency standards, a further reduction in the pressure shifting of the output frequency is desired.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved atomic resonance gas cell.

One feature of the present invention is the provision of an atomic resonance gas cell having double partitioning end wall structures defined by an inner partition, facing the atomic resonance gas, and an outer partition, facing the outside atmosphere, with the space between the partitions being evacuated to the subatmospheric pressure, whereby pressure changes in the atmosphere surrounding the gas cell are not transmitted through the end wall structures to the atomic resonance vapor within the gas cell.

Another feature of the present invention is the same as the preceding feature wherein the inner and outer end wall partitions are physically supported essentially only from their marginal edges.

3,510,758 Patented May 5, 1970 Another feature of the present invention is the same as any one or more of the preceding features wherein the inner end wall partition is sufficiently thinner than the outer end wall partition such that inner partition will fail structurally in the presence of atmospheric pressure in the region between the inner and outer partitions, whereby the gas cell is rendered inoperative rather than permit pressure fluctuations in the ambient atmosphere to be transmitted to the atomic vapor.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of an atomic resonance gas cell incorporating features of the present invention.

FIG. 2 is a transverse view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows, and

FIG. 3 is a schematic block diagram of an atomic resonance frequency standard incorporating fetaures of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, there is shown the atomic resonance gas cell 1 incorporating features of the present invention. The gas cell 1 includes an optically transparent microwave permeable envelope structure, as of glass, having a cylindrical side wall portion 2, as of 2.165 inches in inside diameter and 2.230 inches in outside diameter with an axial length of 0.788 inch. A pair of flat end wall structures 3 are sealed over the ends of the side wall portion 2.

The end wall structures 3 each include an inner partition 4 and an outwardly spaced outer partition 5. The gas cell 1 is filled with an alkali metal vapor as of rubidium or cesium intermixed in a buifer gas to a total pressure of approximately 8 torr. A suitable buffer gas is a mixture of argon and neon comprising 45 percent argon and 55 percent neon for buffering rubidium 87 vapor. The rubidium metal is preferably coated on the inside of an axially directed tubular extension 6 of the envelope which communicates with the interior of the gas cell '1 and forms a rubidium reservoir. The extension 6 is sealed to the cylindrical side wall 2 and to the end wall partitions 4 and 5. At the operating temperature of the gas cell 1, such as for example C., the rubidium metal clings to the inside wall of the reservoir portion 6 and its vapor pressure is such that it serves as a reservoir for rubidium vapor within the gas cell 1.

The inner partition 4 faces the gas fill of the envelope 1 and is disc shaped. The inner partition 4 is sealed essentially only at its outer marginal edge to the cylindrical side wall 2. The outer partition 5 faces the outside atmosphere and is sealed essentially only at its outer marginal edges to the cylindrical side wall 2.

The inner partition 4 is axially spaced from the outer partition 5, as by approximately 0.039 inch, to define a space 7 therebetween which is evacuated to a subatmospheric presusre, as of 10- torr to A of an atomsphere, by means of exhaust tubulation 8 which is sealed off after evacuation of region 7. In a preferred embodiment, the inner partition 4, as of 0.039 inch thick, is substantially thinner than the outer partition 5, as of 0.079 inch thick, such that if the evacuated region 7 therebetween develops a leak and is let up to atmospheric pressure, the inner partion 4 will have insuificient structural strength to hold off the atmospheric pressure and will fracture rendering the gas cell 1 inoperative. The evacuated region 7 serves to prevent atmospheric pressure from being transmitted through the end wall structures 3 to the gas within the gas cell 1.

In the typical prior art gas cell, not shown, which incorporated only single end wall partitions, the pressure coeflicient, for a gas fill as aforedescribed, was approximately 3 X 10- per inch of mercury, whereas the gas cell .1, incorporating the double end wall partitions 4 and 5 with the evacuated region 7, provides, with the same gas fill, a pressure coefiicient of approximately 5 10 per inch of mercury. For the prior art cell having outwardly domed end walls the pressure coefiicient, with the aforedescribed gas fill, was approximately 4 10- per inch of mercury. Thus, it is seen that the gas cell 1 of the present invention provides two orders of magnitude enhancement in the pressure coefiicient as compared to the prior fiat ended cells and approximately one order of magnitude enhancement compared to the prior domed end wall cell.

In use, the gas cell 1 is disposed within a microwave cavity resonator, more fully described with regard to FIG. 3, such that the microwave fields at the atomic resonance frequency are coupled through the walls of the gas cell 1 into the ensemble of atomic resonance vapor within the cell. Optical pumping radiation is passed axially through the cell 1 for raising the ensemble of alkali metal vapor to a nonequilibrium energy state from which microwave resonance transitions may be induced to obtain microwave atomic resonance of the vapor. In a frequency standard, the cavity is tuned for resonance at the resonance frequency of a hyperfine field independent transition and microwave resonance of the gas is detected to derive a frequency standard output.

Referring now to FIG. 3, there is shown a rubidium frequency standard incorporating the gas cell 1 of the present invention. The frequency standard of this type is disclosed and claimed in U.S. Pat. 3,159,797 issued Dec. 1, 1964 and assigned to the same assignee as the present invention. Briefly, the frequency standard includes a gas cell 1 filled with gas, as aforedescribed, and disposed within a cavity resonator 15. The cavity resonator 15 is tuned to the field independent atomic resonance frequency of the alkali vapor such as rubidium 87 within the gas cell 1. For rubidium 87, a suitable field independent hyperfine transition is the (F=2, m=) (F=1, m=0) transition having a resonance frequency at about 6.8 gHz. The end walls of cavity resonator 15 are perforated at 16. Optical pumping radiation, derived from a rubidium lamp 18 and filtered by a rubidium 85 filter 17 to remove unwanted resonance radiation, is passed axially through the gas cell 1 and cavity end walls 16 to a photodetector 19.

The gas cell 1 and filter cell 17 are maintained at a constant desired temperature as of 85 C. by means of an oven 21 which envelops the gas filter cell 17 and cavity 15. The oven 21 includes a thermally conductive box or enclosure 22 with heating coils 23 mounted in heat exchanging relation therewith for maintaining the oven at the constant temperature. A magnetic shield structure 24 encloses the oven 21 and includes, for example, three concentrically disposed magnetic shielding enclosures 25, 26 and 27, as of Molypermaloy, for reducing the stray magnetic fields within the gas cell 1 to a low intensity as of less than 100 microgauss. A magnetic solenoid structure 28 is disposed inside the magnetic shield 24 for producing an axially directed magnetic field H within the gas cell 1 of a predetermined intensity greater than 100 microgauss for separating the magnetic field dependent atomic resonance lines from the desired magnetic field independent resonance line.

The cavity resonator 15 is excited with microwave energy, at the atomic resonance frequency of the atomic vapor Within the gas cell 1, to induce field independent transitions of the atomic vapor. In the case of rubidium 87, the microwave energy is nominally at the frequency of 6.8 gHz. and is derived from a megahertz oscillator 31 by means of a frequency synthesizer 32 and a pair of multipliers 33 and 34, respectively, forming a multiplier chain.

The microwave energy applied to the cavity 15 is frequency modulated at a conveniently low modulation frequency, as of 107 hertz, by means of a frequency modulator 36 feeding one output to multiplier 34 and another out put to a phase detector 37. When the microwave energy applied to the cavity 15 is precisely at the field independent atomic resonance frequency of the vapor within the gas cell 1, the output of the photodetector 19, which monitors the transmitted optical pumping radiation which is passed through the gas cell 1, will contain an intensity modulated signal component at the 107 hertz modulation frequency. This fundamental modulation component is amplified in amplifier 38 and fed to one input of the phase sensitive detector 37 wherein it is compared with the modulation signal derived from modulator 36 to produce a low frequency error signal having a phase and magnitude corresponding to the sense and degree that the frequency of the microwave energy applied to the cavity 15 differs from the precise atomic resonance frequency of the field independent transition.

This error signal is fed to the 5 megahertz oscillator 31 for correcting the frequency of the microwave energy such that it is precisely centered at the field independent atomic resonance frequency. Thus, the frequency of the 5 megahertz oscillator 31 is phase locked to the atomic resonance line such that the 5 megahertz output of oscillator 31 is precisely controlled by the atomic resonance of the alkali metal vapor. Additional frequency standard outputs are derived from the 5 megahertz oscillator by means of dividers 41 and 42, respectively, to produce frequency standard outputs at l megahertz and kilohertz, respectively. Use of the double wall gas cell, as aforedescribed, causes the frequency standard outputs to be free of pressure induced changes to better than 5X10" parts per inch of mercury.

Although the double end wall structure for the gas cell 1 has been described employing a cylindrical side wall structure, this is not a requirement and the double end wall structure may be applied, if desired, to gas cells employing domed end wall structures which may or may not include a cylindrical side wall portion.

As in the prior art gas cells having cylindrical side walls, the cylindrical side wall 2 is sufiiciently rigid such that it produces only a negligible pressure shift in the resonant frequency of the atomic vapor.

The term atmosphere is used herein to describe the surrounding gaseous environment in which the gas cell 1 is immersed. It is contemplated that the gas cell 1 will be employed in certain airborne or space borne applications where the atmospheric pressure can vary from sea level pressure to essentially zero pressure as encountered in outer space. However, the thin inner partition 4 is dimensioned to fracture when a pressure differential across the partition is obtained which is equal to that obtained when the pressure inside the cell is less than 100 torr and the pressure in the space 7 reaches sea level pressure.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a gas cell for atomic resonance apparatus, means forming a gas tight radio frequency wave permeable dielectric envelope, an ensemble of gaseous atoms contained within said envelope, said envelope having an optically transparent portion through which said gaseous ensemble is to be illuminated with optical pumping radiation, said envelope having a pair of mutually opposed end wall structures, the improvement wherein, said end wall structures each include an inner partition facing the enclosed gaseous ensemble and an outer partition facing the outer atmosphere, said outer partition being outwardly spaced from said inner partition to define a region between said partitions which is evacuated to subatmospheric pressure, whereby atmospheric pressure stresses are not transmitted through said double end wall structure to said ensemble of gaseous atoms contained within the gas cell, said inner end wall partition being sufiiciently thinner than said outer end wall partition such that said inner partition will fail structurally in the presence of atmospheric pressure at sea level in the region between said inner and outer partitions when the pressure within the inner partition is below 100 torr.

2. The apparatus of claim 1 wherein said inner and outer end wall partitions are physically supported essential- 1y only from their marginal edges.

3. The apparatus of claim 2 wherein said envelope includes a cylindrical side wall structure, and said end Wall structures being mounted over the ends of said cylindrical side wall structure.

4. The apparatus of claim 1 including a buffer gas contained within said envelope, said ensemble of gaseous atoms being an alkali metal vapor intermixed with said buifer gas.

UNITED STATES PATENTS 2,882,493 4/1959 Dicke 33 l3 3,159,797 12/1964 Whitehorn 33194 3,242,423 3/ 1966 Malnar 3240.5 3,248,666 4/1966 Farmer 331-3 3,308,394 3/1967 Snitzer 33194.5 3,418,565 12/1968 Broussaud 324-0.05

RUDOLPH V. ROLINEC, Primary Examiner M. J. LYNCH, Assistant Examiner US. Cl. X.'R. 3313, 94 

